1. Field of the Invention
The present invention relates to an improved apparatus (hardware arrangement) and algorithm that employs a direct digital radio frequency synthesizer to generate and transmit multiple simultaneous radio frequency (RF) beams, which can be electronically scanned.
2. Description of the Prior Art
In the past, a number of methods and apparatus have been used to generate electronically scanned radio frequency beams using array elements. These methods include both analog beamforming and digital beamforming techniques that are applied to transmit array antennas as discussed in the references [1,2]. By the principal of superposition it is possible to apply same frequency signals to each of N radiating elements so that the summation of same frequency signals at a point in the field of the elements forms a single beam, which can be electronically scanned by introducing a relative timing or phase delay at each of the elements. Those skilled in the art know that for example, the aperture size of an array antenna can be increased by increasing the total number of radiating elements N so that a sufficiently narrow beam width can be achieved so as to direct RF energy in a specified direction with a desired beam width. It is possible to digitally generate RF signals using an apparatus referred to a direct digital synthesizer (DDS) and as described in the references [3,4,8,9] for example. Such DDSs produce RF signals with an output that is determined by digital control signals, which may include clock, amplitude, frequency and phase control signals. Using a DDS it is therefore possible to generate a RF waveform which is defined by the digital control signals.
It is possible, therefore, to use a DDS to generate a digitally formed beam that can be electronically scanned with digital control signals. Prior art methods for digitally forming RF beams using a DDS are disclosed, for example, in references [6,6a,7]. In the prior art, an architecture produces a single RF beam with N element chains, where an element chain consists of at least a DDS, digital control signals, and a radiating antenna element. The digital control architecture, amplitude, phase and frequency are all provided in parallel to each of the elements, thus facilitating a means of controlling modulation of the produced waveform. A clock signal is distributed to DDS circuits in order to establish a timing reference useful in synchronizing multiple DDS circuits. Proper phasing of the RF waveforms provided to each of the radiating elements within the array permits the beam to be electronically scanned to a desired pointing direction. The limitation of the architecture of the prior art is that it produces only a single beam per beamformer and array, using the N elements.
It is often desirable to radiate more than one RF beam, where each beam or set of beams may have a different center frequency and modulation using the same array aperture as disclosed in references [1,2,5,5a]. However, the existing prior art method for generating M multiple simultaneous beams from the same aperture is to use the principle of superposition to sum signals prior to driving the radiating element, so that it requires M beamforming units to combine the signals. As disclosed in references [6,6a,7], one DDS circuit is required for every radiating element in order to produce a single RF beam. Thus, In order to handle M different information signals that are to be formed into M different independent RF beams, M DDS circuits are required per radiating element in the antenna array. The synthesized RF signals for a given radiating element (or subarray) are summed using an RF combining network and applied to the radiating element. The RF signals are radiated and by superposition form the desired RF beams. Therefore, prior art systems, would require Nxc3x97M total DDS circuits to form M beams having the same beamwidth as the single beam system described above. Thus, there is a need in the prior art to reduce the hardware requirements for a DDS electronically scanned array system that is configured to farm multiple independent beams in an improved manner.
Accordingly, a principal object of the present invention is to configure an apparatus and create a method that efficiently and optimally produces multiple simultaneous RF beams, i.e., M beams, each of which can have a different frequency and separate modulation, are independently electronically scanned and can be easily defined in software.
A corollary of the above object is to reduce the number of DDS circuits needed from Nxc3x97M to N, while maintaining the effective aperture size used to generate each of the M independently pointed beams which may contain independent frequency and/or modulation information.
A further object of the present invention is to reduce the need for RF combining circuits in order to generate the multiple simultaneous RF beams.
Another object of the present invention is to generate additional RF beams, which can be used to transmit radar, communications, or other information with a minimal increase in hardware.
Yet still another object of the present invention is to improve the capacity to generate additional RF beams by defining in software the number, frequency, and modulation of the RF beams thereby providing further improvement in system flexibility.
In accordance with the above stated objects, other objects, features and advantages, the apparatus of the present invention is configured to generate multiple simultaneous RF beams using a minimal number of DDS circuits and related hardware. This increase in the number of RF beams in an antenna array system permits an increase in throughput of radar, communications and other signal information without increasing the DDS circuits necessary, and, accordingly, the number of associated radiating elements in the antenna array system needed.
The essence of the invention is in the use of digital signal processing to combine signals digitally so that the same DDS per radiating element can produce the superposition of multiple waveforms to create the multiple beams. The digital control signals that are applied to each DDS consist of amplitude, phase, and frequency signals, which are parameters that can be defined in software. The apparatus and method of the present invention are improvements over the prior art since they require less circuitry and provide the capacity to reconfigure the number of RF beams without an increase in hardware.